The usual transmitter types are non-linear AM transmitters featuring a radio frequency (RF) branch and an amplitude (A) branch. For reasons of higher efficiency, the RF branch and the amplitude branch operate in switched mode. In the output stage of the transmitter, the RF signal and the amplitude signal are multiplied by each other, which is also carried out in switched mode for reasons of efficiency. The switched mode is controlled by the RF signal. In contrast, for the amplitude signal, the input of the multiplier is linear.
FIG. 2 illustrates a modulated digital signal generated by two partial signals (the in phase (I) signal and the quadrature (Q) signal, which are orthogonal to each other. The I signal is modulated onto a cosine oscillation having the frequency Ft (carrier frequency). The Q signal is modulated onto a sine oscillation having the same frequency Ft. The sum of both modulated oscillations produces the complex modulated data signal (cosine 0–180degrees, sine −90–+90degrees). The modulated I/Q signal is shaped by filters in such a manner that it has exactly the prescribed curve shape with the desired bandwidth.
For digital broadcasting, the modulated I/Q signal is converted in such a manner that the two signals, amplitude (A) signal and phase-modulated carrier signal (RF-P signal), result therefrom which are suitable for proper control of the AM transmitter. Then, at the output of the AM transmitter, the modulated I/Q signal is generated again with higher power.
To minimize out-of-band and spurious emissions, it is required for the A signal and the signal to be present in the transmitter output stage at the same time. As a result of this, the delay between the two signals has to be compensated for and the remaining difference must not be greater than 0.3 microseconds, given a channel bandwidth of 9–10 KHz, since the permissible delay difference is reciprocally proportional to the channel bandwidth.
Since both the A-signal and the RF-P signal have a considerably larger bandwidth than the I/Q signal, the increased bandwidth should be available within the transmitter, which, however, cannot be achieved to a sufficient degree due to technical and economic reasons. The insufficient bandwidths result in unwanted out-of-band and spurious emissions which have to be minimized.
From German patent document DE 10112025.7, it is known that the bandwidth of the A-signal and RF-P signal can be reduced if the vector diagram of the I/Q signal is provided with a hole around the 0/0 point. The larger the hole in the vector diagram of the I/Q signal, the lower are the gradients of the out-of-band and spurious emissions in the spectrum. This connection is given by the fact that, spectrally, the gradients of the out-of-band and spurious emissions due to the delay differences correspond to the gradient of the RF-P signal, and are able to be partially corrected by the hole in the vector diagram. The hole in the vector diagram cannot be made as large as desired, because otherwise the modification of the signal appears as a disturbance.
It is also known that the shoulder distance of the out-of-band and spurious emissions depends both on the magnitude of the delay difference and on the available bandwidth in the amplitude branch and also in the RF branch of the transmitter. It follows therefrom that the delay difference should go toward zero, and that the bandwidth in the branches should be as large as possible.
Therefore, it is desirable to implement as large a bandwidth as possible for both branches in order to further reduce the out-of-band and spurious emissions of the AM transmitter. For the amplitude branch, it is possible to achieve a linear phase response and, thus, a constant delay up to the frequency limit by equalization in terms of delay.
The phase-modulated RF-P signal, just as a frequency-modulated signal, has an infinite number of lateral spectral lines so that the bandwidth required for transmission would theoretically have to be infinitely large as well. Since the weighting of the spectral lines decreases rapidly, it can be achieved by using a suitable filter that only a minimum of distortions occurs because of the limited bandwidth. It is known that this condition is satisfied by a filter having a Gaussian transfer function.